1. Field of the Invention
The present invention relates to an image formation apparatus that is used as an output unit for outputting multi-value image data that has been generated by a computer or the like. Particularly, the invention relates to an antialiasing technique for improving the picture quality that makes it possible to decrease the zigzag effect of a jaggy (a zigzag of pixels (dots)) that is generated in the image formation of characters or line drawings. The present invention can be applied to any kind of device that can form an image of at least one density in addition to the two values of black and white in pixel units. Examples of the application of the present invention to a printer, particularly, a laser printer, will be explained.
2. Description of Related Art
FIG. 1 is a diagram that shows an example of a structure of a laser beam printer.
A reference number 1 denotes a main body of a laser beam printer. The main body has an optical section 2 and an image formation section 11. The optical section 2 includes a laser 3 that outputs a laser beam, a polygon mirror 4 that deflects a laser beam so as to repetitively scan the laser beam, a mirror motor 5 that rotationally drives the polygon mirror 4, and abeam detector 6 that detects a start of the scanning of a laser beam. The image formation section 11 includes a photosensitive drum 7, a developer 8, a transfer roller 10, and a return mirror 9. The surface of the photosensitive drum 7 is charged with an electric charge, not shown. Thereafter, the photosensitive drum 7 receives a laser beam on this surface. A laser beam signal from the laser 3 is modulated in synchronism with the scanning of the laser beam and the rotation of the photosensitive drum 7. Then, an optical image corresponding to an image to be printed is drawn on the surface of the photosensitive drum 7. Static electricity charged onto the surface of the photosensitive drum 7 decreases according to the intensity of the laser beam, so that a static latent image is formed on the surface. When a charged toner is contacted to the surface of the photosensitive drum 7 by the developer 8, the toner adheres to the surface of the photosensitive drum 7 according to the static latent image so that an image is formed. A sheet of paper is brought into contact with the surface of the photosensitive drum on which the toner image has been formed. The transfer roller 10 rolls the sheet on the surface of the photosensitive drum to transfer the toner image onto the paper. A fixer not shown then fixes the transferred toner image onto the paper. As a result, the printing of the image onto the paper has been completed. After the transfer of the toner image, the surface of the photosensitive drum is cleaned. The surface of the photosensitive drum is charged with an electric charge again, and the above process is repeated.
The print data that has been input from a computer system is expanded as a print image data and is stored in an image memory 12 by an image expander 13. In general, the image memory 12 is called a bit map memory. In the case of two-value print data, each bit of the bit map memory, that is, one bit, corresponds to a print pixel. In the case of a multi-value print data, a few bits of the bit map memory correspond to a print pixel. For example, in the case of a four-bit print data, the bit map memory also has a four-bit structure. Based on the four-bit print data, it is possible to express 16 gradations in pixel units. An optical modulation signal generator circuit 15 reads out the image data stored in the image memory 12 in synchronism with the processing of the main body, and generates an optical modulation signal and applies it to the laser 3.
In the single-color two-value printers such as monochromatic laser printers, there has been partly employed a picture-quality improvement processing (a smoothing processing). According to this method, a jaggy that has been generated in the printing of characters and line drawings is automatically judged from the print bit map data expanded on the image memory. The jaggy is then converted into a data of higher resolution than that of the input image data, thereby to decrease the zigzag effect of the jaggy.
FIG. 2A to FIG. 2C are diagrams for explaining an example of the smoothing processing of the two-value image data. As shown in FIG. 2A, it is assumed, for example, that there are vertical lines each of which is deviated by one pixel (dot) position in the middle of the line in the print image data stored in the image memory 12. This deviation is a jaggy. In the smoothing processing, the existence of a jaggy is decided based on a matching of patterns of pixel data including neighboring pixels. At the jaggy pixel positions of the lines, the output timing of the optical modulation signal is adjusted as shown in FIG. 2B. The image is printed after this adjustment. As shown in FIG. 2C, the jaggy portions of the vertical lines are smoothed by decreasing the size of the deviation by a half dot. The above shows an example of a smoothing adjustment carried out based on a half-dot adjustment in the same direction. However, it is also possible to adjust the dots in any direction of left and right. In other words, the smoothing processing is an addition or a deletion of dots at jaggy portions to make it possible to print an image by increasing the resolution of the original image data by an integer times in the main scanning direction of the laser beam.
FIG. 3 shows one example of a structure of a picture quality improvement circuit that carries out a smoothing processing of a two-value print data. As shown in FIG. 3, an optical modulation signal generator circuit 15 has an image memory reader 16 and a picture quality improvement circuit 17. Further, the picture quality improvement circuit 17 has a line buffer 18, an evaluation window extractor 21, and a correction signal generator 22.
The image memory reader 16 reads out from an image memory 12 bit map data that is located a few lines before the print data currently under exposure by the laser 3, and transfers the read-out data to the line buffer 18. The line buffer 18 consists of a shift register which holds data of a few lines before and after the print data currently under exposure.
The evaluation window extractor 21 extracts data of a rectangular area (this area is called an xe2x80x9cevaluation windowxe2x80x9d) 19 around one focused pixel (this pixel is called a xe2x80x9cfocused pixel (dot))xe2x80x9d in the data that is being held in the line buffer 18. Then, the evaluation window extractor 21 outputs an extracted-pattern layout signal that expresses a pixel layout in the rectangular area. This extracted-pattern layout signal is input to a correction signal generator circuit 22. The correction signal generator circuit 22 is a circuit that generates a correction signal for a focused pixel based on a dot layout within the evaluation window shown by the extracted-pattern layout signal. The correction signal generator circuit 22 has a look-up table that stores a neighboring pattern of the focused pixel. The correction signal generator circuit 22 correlates the input extracted-pattern layout signal with the look-up table, and outputs a correction signal that has been stored based on this correlation. For example, when it is not necessary to correct the focused pixel, the correction signal generator circuit 22 generates a modulation signal based on the original pixel data, and outputs this modulation signal as a correction signal. When it is necessary to correct the focused pixel, the correction signal generator circuit 22 generates a correction signal based on a correction data stored in advance, and outputs this correction signal.
FIG. 4A to FIG. 4D are diagrams for explaining the smoothing processing of the dot layout pattern of the image data shown in FIG. 2A to FIG. 2C. FIG. 4A shows a pattern of an extracted rectangular area that has been extracted in the size of 5 times 5 pixels around a xe2x80x9cfocused pixelxe2x80x9d including this xe2x80x9cfocused pixelxe2x80x9d which is located at a position of a line=N and a pixel=Mxe2x88x921. When it has been judged that the pattern stored in the look-up table coincides with the pattern of the extracted square area, a decision is made that it is necessary to correct the pixel which is located at the position of the line=N and the pixel=Mxe2x88x921. As a result, a correction data as shown in FIG. 4B is output. An optical modulation signal is then generated based on this correction data. As explained below, correction data is also generated for the pixel located at a position of the line=N and a pixel Mxe2x88x922, and an optical modulation signal is generated based on this correction data.
FIG. 4C shows a pattern of an extracted square area that has been extracted in the size of 5 times 5 pixels around a xe2x80x9cfocused pixelxe2x80x9d including this xe2x80x9cfocused pixelxe2x80x9d which is located at a position of a line=N and a pixel=M+1. When it has been judged that the pattern stored in the look-up table coincides with the pattern of the extracted rectangular area, a correction data as shown in FIG. 4D is output as a correction data at a position of the line=N and the pixel=M+1. Based on this, an optical modulation signal is generated.
The picture quality improvement circuit shown in FIG. 3 sequentially moves in synchronism with a pixel printing timing of the xe2x80x9cfocused pixelxe2x80x9d in the main body 1. The picture quality improvement circuit then outputs an optical modulation signal at a position of the xe2x80x9cfocused pixelxe2x80x9d at the time of actually printing the pixel at the position corresponding to the xe2x80x9cfocused pixelxe2x80x9d. A control clock signal generator 23 within a control circuit 14 generates a control clock for controlling the operation timing of the picture quality improvement circuit.
The above explains the case that a pixel is corrected by increasing the resolution of the pixel by two times in the main scanning direction. When a pixel is to be corrected by increasing the resolution of the pixel by three times, a similar operation is conducted. In the present specification, in order to simplify the explanation, there will be explained only the case where a pixel is corrected by increasing the resolution of the pixel by two times in the main scanning direction.
There have also been provided multi-value printers that can express pixels in the intermediate densities in print pixel units by using multi-values in addition to the two values of white and black. For example, according to the laser beam printer as shown in FIG. 1, the light-emitting quantity or the light-emitting time of the laser 3 of the optical section is controlled to change the size of print pixels formed by the image formation section, thereby to equivalently change the densities of the print pixels.
FIG. 5 is a diagram showing one example of a structure of a laser beam printer that carries out a multi-value printing in print pixel units by changing the light-emitting quantity of the laser. In FIG. 5, the laser beam printer has a main body and a control circuit that have substantially the same structure as those of the main body 1 and the control circuit 14 shown in FIG. 1. Multi-value print bit map data having density gradations expanded to an image memory 31. An image memory reader 33 reads out a pixel data from the image memory 31 in synchronism with a pixel printing timing in the image formation section. A D/A converter 34 converts the pixel data read out from the image memory 31 into an optical modulation signal having an analog intensity corresponding to the multi-value data according to the density values of the pixel data.
A smoothing processing is also carried out in the laser beam printer that conducts a multi-value printing. FIG. 6A to FIG. 6C are diagrams showing an example of a smoothing processing of four-value image data. FIG. 6A shows one example of a pattern of four-value bit map data on the image data memory. FIG. 6B shows an optical modulation signal at the time of multi-value printing an image in print pixel units by changing the light-emitting quantity of the laser. FIG. 6C shows a print result.
For carrying out a smoothing processing of multi-values shown in FIG. 6A to FIG. 6C, a picture quality improvement circuit that has a similar structure to that shown in FIG. 3 is also used. The only difference is that the picture quality improvement circuit in this case processes the multi-value data as pixel data. Therefore, for this purpose, it is necessary that the line buffer 18 can store multi-value data, that the evaluation window extractor 21 can extract an extracted-pattern layout signal of the multi-value data and can transfer this signal to the correction signal generator 22, and that the correction signal generator 22 has a multi-value look-up table. However, when the number of densities including white that can be expressed in print pixel units is C, the number of combinations of patterns that are extracted as evaluation windows becomes (C/2)25 times the number of combinations in the case of the two-value data Further, the size of the look-up table becomes D times in order to express the C densities of the output of the correction signal. This D is an integer that meets the condition of 2Dxe2x88x921 less than Cxe2x89xa62D.
For example, when the number of idensities including white that can be expressed in the print pixel units is four, the size of the look-up table becomes (4/2)25xc3x972=67,108,864 times the number of densities in the case of the two-value data. In this case, the number of combinations that are input to the look-up table is the number of combinations of the patterns that are extracted as evaluation windows. This number becomes 35,554,432 times the number of combinations of the patterns in the case of the two-value data.
As explained above, when the picture quality improvement circuit of the conventional technique is directly applied to the picture quality improvement (smoothing) of a multi-value image in the multi-value printer, the necessary size of the look-up table becomes enormously large. This has a problem that the preparation of the table data becomes very complex because the table data needs to be the combination of the pixel layout and the pixel densities, which is not practicable.
It is an object of the present invention to provide an image formation apparatus capable of achieving a picture quality improvement of a multi-value image without increasing the required size of a look-up table to a size larger than the size of a look-up table that is used for the picture quality improvement at the time of forming two-value image data.
In order to achieve the above object, according to the present invention, there is provided an image formation apparatus which distributes an input multi-value image data to a plurality of density planes based on a predetermined distribution rule according to densities that can be expressed in pixel units, carries out a picture quality improvement processing for each density plane, outputs correction signals produced as a result of the picture quality improvement processing for each density plane, and combines these correction signals according to a predetermined combining rule, thereby to correct the layout of the pixels.
In other words, the image formation apparatus of the present invention is an image formation apparatus that can form an image of at least one density in addition to two values of white and black in pixel units, the image formation apparatus comprising: a density separation section that distributes a multi-value image data to a plurality of density planes according to a predetermined rule; a correction signal generator that outputs correction signals for correcting the layout of pixels in each of the plurality of density planes; and a correction signal combiner that combines correction signals output from the density planes according to a predetermined combining rule.
When the number of gradations that can be expressed in pixel units is C, for example, it is preferable that the number of the plurality of density planes of the density separation section is Cxe2x88x921. However, the number may be smaller than this so long as the number is a plural number.
According to a first aspect of the present invention, the density separation section distributes a multi-value image data to a plurality of density planes according to a distribution rule for distributing the multi-value image data based on gradations that can be expressed in pixel units. According to a second aspect of the invention, the density separation section distributes the multi-value image data to a plurality of density planes according to a distribution rule for distributing the multi-value image data based on threshold values of gradations that can be expressed in pixel units.
When there are a plurality of density planes for the same pixel, the correction signal combiner combines correction signals according to a combining rule of selecting and outputting a correction signal that is output from the density plane having the highest density. This means the following cases when there are a plurality of density planes for a certain pixel. If for example, a correction has not been carried out in the density plane of the highest density, and the original pixel data has been converted into a correction signal without correction, the original pixel data is output as it is as an optical modulation signal even if a correction has been carried out in other density plane of which density is lower than that of this original pixel data. On the other hand, if a correction has been carried out in a high-density plane, an image is formed according to a correction signal of this high-density plane, even if a correction has not been carried out in other lower-density plane. The correction signals are combined in pixel units.
According to the present invention, the image data distributed to each density plane can be handled as a two-value image data. It is, therefore, possible to achieve the picture quality improvement of a multi-value image in the apparatus by directly using the look-up table that is used for picture quality improvement at the time of forming two-value data.
In order to achieve the above picture quality improvement (smoothing processing), it is necessary that the printer can form a multi-value image at a resolution an integer times the resolution of the pixels of the input image, in at least one direction, that is, in the main scanning direction in the case of a laser beam printer. In other words, each pixel of the image data consists of several apparatus pixels, in at least one direction.
With a suitable pixel of an image to be formed selected as a focused pixel, there is used an evaluation window extractor that extracts a predetermined area covering this focused pixel together with its neighboring pixels and that outputs extracted-pattern layout signals for expressing pixel layout positions of density planes. The correction signal generator generates correction signals based on the extracted-pattern layout signals.
Therefore, each density plane may have a capacity capable of expanding a predetermined area covering the focused area and its neighboring pixels within a minimum evaluation window. There may be also provided a data memory as an option that temporality stores data of the plurality of density planes distributed by the density separation section. In this case, the data memory may have a capacity capable of temporarily storing a few lines of bit map data of the image memory.
In any case, each of the plurality of density planes may have a small memory capacity that can temporarily store either a predetermined area covering the focused area that is a pixel corresponding to a pixel position in the image to be formed and its neighboring pixels or a few lines of data including the focused pixel.